I. Field of the Invention
The present invention generally relates to wireless communications. More specifically, the present invention relates to a method of and apparatus for providing short message services in different cellular communications systems.
II. Description of the Related Art
Code division multiple access (CDMA) modulation is but one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA), the global system for mobile communications (GSM) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are also available, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters”, assigned to the present assignee, the disclosure of which is incorporated herein by reference.
In U.S. Pat. No. 4,901,307 a multiple access technique is described in which a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as cell base stations, or cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
In conventional cellular telephone systems the available frequency band is divided into channels typically 30 KHz in bandwidth while analog FM modulation techniques are used. The system service area is divided geographically into cells of varying size. The available frequency channels are divided into sets with each set usually containing an equal number of channels. The frequency sets are assigned to cells in such a way as to minimize the possibility of co-channel interference. For example, consider a system in which there are seven frequency sets and the cells are equal size hexagons. A frequency set used in one cell will not be used in the six nearest or surrounding neighbors of that cell. Furthermore, the frequency set in one cell will not be used in the twelve next nearest neighbors of that cell.
A more difficult situation is presented by movement of the mobile station into a cell serviced by a base station from another cellular system. One complicating factor in such “intersystem” handoffs is that the neighboring cellular system often has dissimilar characteristics. For example, adjacent cellular systems will often operate at different frequencies, and may maintain different levels of base station output power or pilot strength. Further, adjacent cellular systems may require different messaging structures, even for similar types of messages.
For example, the so-called GSM standard has no mechanism for a soft handoff. There is, therefore, a problem in handing off a call using the air interface from a CDMA network to a GSM network. Furthermore, GSM authentication cannot be done because the CDMA 2000 mechanisms cannot transfer the data required to do GSM authentication. Encryption in GSM is different than the encryption in CDMA 2000.
In another example, there is a problem in that there is currently no mechanism for handing SMS and SMSC information using the air interface from a CDMA network to a GSM network. That is, when an SMS message is sent from the mobile, the network needs to know the SMSC address to send the SMS message. Similarly, standard GSM SMSC information cannot be transferred because the CDMA mechanisms cannot transfer the data required to handle GSM SMSC address information. Moreover, standard CDMA SMS message formats (IS-637) cannot be handled by a standard GSM network, because GSM mechanisms cannot transfer the data required to handle standard CDMA SMS messaging.
One way of dealing with this problem would be to modify GSM to enable it to effect handoff to a non-GSM system, e.g. a CDMA system. Another way of dealing with this problem is to modify CDMA to handle traditional SMSC messaging. However, both CDMA and GSM are well established systems, relatively speaking, and operators and equipment providers are reluctant to make expensive modifications to existing equipment in order to accommodate a neighbouring incompatible system. If new messages are added to the air interface in support of dual-mode mobile stations, then modifications must be made to support these new messages using existing hardware. Plainly, this is undesirable form the perspective of the operator and of the equipment provider.